After spinning turbines to generate power, the water is sent through a smaller line to the city of Everett water treatment plant at Lake Chaplain.

About every 10 years, the big tube is emptied out and inspected from the inside to make sure it’s in good shape. Now is one of those times — it was last done in 2003.

So far, so good, PUD officials said Wednesday after several days of scrutiny.

On Sept. 25, crews cut off the flow of water into the tunnel and pipeline. It took about three days to sufficiently empty the passageways so crews could get inside and look, said Barry Chrisman, superintendent of the hydroelectric plant.

Employees walked the entire four-mile length of the tunnel on Tuesday, from the dam, which was built in 1965, to a portal where the tunnel and pipeline meet.

Ultrasound equipment was used at different points along the way to measure the thickness of the walls to make sure the lining is not wearing away. So far the walls have retained their original thickness, officials said.

Crew members for the PUD and consultants also climbed down into the pipeline, which runs another four miles, via some of the 14 manhole-like portals along the way. Unlike the tunnel, which in some places is hundreds of feet below the surface, the pipeline is eight to 15 feet underground.

Crews checked various parts of the pipeline, also for thickness and any other defects. It also has tested well, officials said.

The tunnel section, with an arched, 14-foot ceiling and flat floor, was dug underneath a mountain with a boring machine and completed in 1983.

At the four-mile mark, it narrows into the round pipeline, 10 feet in diameter. The pipeline runs the rest of the way to the powerhouse.

The tunnel and pipeline are usually packed with moving water, exerting 100 pounds per square inch of pressure outward on the walls. Those walls are made of a layer of vinyl on the inside and outside, sandwiched around a layer of steel.

When its running at its fastest, an atom of water travels the eight miles from the dam to the powerhouse in just over 40 minutes.

In the early 1980s, Culmback Dam was raised and the pipelines and powerhouse built for a combined $250 million, Chrisman said.

At the powerhouse, the water spins four turbines that supply power to an average of 35,800 homes — equaling roughly 5 percent of Snohomish County’s electricity.

Some of the water is emptied into the Sultan River. The rest is sent to Lake Chaplain, making up 80 percent of Snohomish County’s drinking water.

When the pipeline was built, a series of traps were built into the bottom to catch rocks and other debris. At the time, it was uncertain how much rock would come down through the tunnel and pipeline. Seven traps were built as a precaution, creating depressions into which the rocks would tumble.

Over the years, fewer rocks were caught in the traps than was originally thought might occur. Not many more rocks were found this time compared to the last inspection a decade ago, officials said.

One of the trap sections was found to be damaged so it was simply removed and not replaced, Chrisman said.

The rock won’t be taken out, he said, because there’s still plenty of room left.

“If we don’t get all of them full, it’s just going to stay there,” Chrisman said.

The water transmission system is expected to be shut down for a total of about 10 days, officials said. During that time, they’re using the opportunity to do other maintenance around the powerhouse that’s difficult or impossible when water and electricity are surging through the plant.

For example, valves are being replaced in pipelines. Meters that measure electricity at the switching station next to the powerhouse have been upgraded. Insulators at the station have been pressure washed.

Normally, workers are required to stay four feet away from structures in the upper reaches of the switching station, which has 115,000 volts running though.

“Otherwise, it’ll reach out and touch you. Only once,” Chrisman said.

The PUD is not required by law to inspect the tunnel every 10 years, but it’s recommended by consultants, officials said.

After this inspection, a consultant will determine whether the PUD can go more than 10 years before the next inspection, or whether it should be done sooner. The vinyl lining in the tunnel and pipeline has a lifetime of 30 to 50 years, and was installed 30 years ago.

The utility doesn’t turn off the water often because it reduces available electricity, officials said. The inspections and repairs also incur extra costs in staff time and contractors. And the water pressing against the walls of the tunnel helps reduce the pressure from the weight of the ground above it.

Crews who did past inspections carefully documented their work, said Bruce Meaker, a principal engineer for the PUD.

“We had the benefit of that,” he said.

It helped the staff streamline the work this time compared to a decade ago, said Chrisman, a 22-year veteran of the PUD.